The present invention is directed to electric power transmission system protection apparatus and, more particularly, to an improved quadrilateral characteristic for a ground distance relay.
Electric power transmission systems frequently employ so-called distance relays to determine whether a fault on the system is within a predetermined distance of a monitoring point where the relay is located. Each predetermined distance is often referred to as a separately numbered zone. The present invention is particularly concerned with distance relays which respond to single phase to ground faults within zone 1, a separate such relay being normally provided for each phase of the polyphase power transmission system.
In a well-known form of a distance relay a tripping signal determination is made by comparing the phases of voltages derived from measurements of the system voltage and current at the monitoring point under fault conditions. For example, referring to FIG. 1, in the so-called mho characteristic relay of the prior art the phases of quantities V-IZ and V.sub.pol are compared where:
V is the phase voltage at the monitoring point; PA1 I is the phase current at the monitoring point; PA1 Z is a replica impedance which determines the setting of the relay, i.e., the predetermined distance or reach; and EQU V.sub.pol is V.angle.-90.degree..
Since the quantities V-IZ and V are necessarily at 90.degree., and hence the quantities V-IZ and V.sub.pol are necessarily in phase when the meeting point of the vectors V and V-IZ lies on a circle having IZ as diameter, by determining, using a phase comparator, whether V-IZ leads or lags V.sub.pol it can be determined whether the fault is beyond or within the distance for which the tip of the vector V lies on the circle.
In addition to the capability of discriminating with respect to distance, it is highly desirable that not all faults or fault combinations within a prescribed distance or zone necessarily lead to the tripping of a breaker. For example, a ground distance relay for phase A preferably should detect phase A-to-ground faults only, not phase B-to-ground faults, or, alternatively, phase-to-phase faults. In other words, it is generally desirable that the relays implemented in an AC power distribution system have a certain degree of discrimination not only with respect to distance, but also with respect to particular fault combinations.
An early attempt to provide a more discriminating ground distance relay is disclosed in U.S. Pat. No. 4,686,601 to the present inventor. That patent discloses a system for improved discrimination between internal and external faults as well as prevention of overreaching by the ground distance relay on single line-to-ground and double line-to-ground faults. That patent further discloses a system to prevent operation of the ground distance relay on a phase other than the phase on which a fault occurs.
U.S. Pat. No. 4,686,601 is based on a mho type characteristic like that discussed above and shown in FIG. 1. However, many protective relay engineers prefer to use a quadrilateral (polygonal) characteristic for ground distance functions to provide more fault resistance coverage than the circular mho characteristic. Further, the system disclosed in U.S. Pat. No. 4,686,601 is not particularly sensitive to preventing the relay function from responding to faults other than single line-to-ground faults on the protected phase. In other words, such a system may respond to any other single line-to-ground fault or any fault that involves more than one phase. Such undesirable relay functions lead to unnecessary tripping of breakers which leads to unnecessary electrical transmission interruptions.